In conventional fluid bed cracking of hydrocarbon feedstocks, it is the practice, because of the rapid loss in catalyst activity and selectivity, to continuously add fresh catalyst regularly, usually daily, to an equilibrium mixture of catalyst particles. If metals, such as nickel and vanadium, are present in the feedstock, they accumulate almost completely on the catalyst, thus drastically reducing activity, producing more undesirable coke and hydrogen, and reducing selective conversion to gasoline. In such cases, catalyst replacement additions may have to rise significantly. Fluid cracking catalysts generally consist of small microspherical particles varying in size from 10 to 150 microns and represent a highly dispersed mixture of catalyst particles, some present in the unit for as little as one day, others there for as long as 60-90 days or more. Because these particles are so small, no process has been available to remove old catalysts from new. Therefore, it is customary to withdraw 1 to 10% or more of equilibrium catalyst containing all of these variously aged particles, just prior to addition of fresh catalyst particles, thus providing room for the incoming fresh "makeup" catalyst. Unfortunately, the 1 to 10% of equilibrium catalyst withdrawn itself contains, 1-10% of the very expensive catalyst added the day before, 1-10% of the catalyst added 2 days ago, 1-10% of the catalyst added 3 days ago, and so forth. Therefore, when removing equilibrium catalyst, unfortunately a large proportion of withdrawn catalyst still represents very active catalyst.
Catalyst consumption can be high. The cost associated therewith, especially when high nickel and vanadium are present in any amount greater than, for example, 0.1 ppm in the feedstock can, therefore, be great. Depending on the level of metal content in feed and desired catalyst activity, tons of catalyst must be added daily. For example, the cost of a catalyst at the point of introduction to the unit can be $2,000/ton or greater. As a result, a unit consuming 20 tons/day of "makeup" catalyst would require expenditures each day of $40,000. For a unit processing 40,000 barrels/day (B/D) this would represent a processing cost of $1/B or 2.5 cents/gallon, for catalyst use alone.
In addition to "makeup" catalyst costs, an aged high nickel and vanadium-ladened catalyst can also reduce yield of preferred liquid fuel products, such as gasoline and diesel fuel, and instead, produce more undesirable, less valuable products, such as dry gas and coke. Nickel and vanadium on catalyst also accelerate catalyst deactivation, thus further reducing operating profits.
Previous means to achieve effective magnetic separation of old catalyst from new is covered in U.S. Pat. No. 4,406,773 (1983) of W. P. Hettinger, et. al, and discloses use of a high magnetic field gradient separator (HGMS) produced by SALA. A carrousel magnetic separator containing a filamentary matrix within produces a high magnetic field gradient to achieve selective separation.